Carbon-carbon composites are widely used for aircraft brake friction materials. Carbon-carbon is attractive because it is lightweight and can operate at very high temperatures, and because it can, pound for pound, absorb a great deal of aircraft energy and convert it to heat. A major drawback with the use of carbon-carbon for this application is the high cost of raw material used to make the parts. Expensive carbon fiber is a significant component; sometimes up to 45% fiber is used in the composite. Fiber costs can often be the single largest contributor to the cost of making a friction material. Another drawback is that the manufacture of carbon-carbon is a time-consuming process. The overall process for making a carbon brake disk is measured in weeks, and even months. Long cycle times are undesirable in a modern manufacturing environment. It is highly desirable to provide a process that has a reduced cost and shortened cycle time for making a carbon-carbon composite.
The invention disclosed herein addresses the major drawbacks of manufacturing carbon-carbon composites: cost and cycle time. These drawbacks have been identified and discussed in the parent U.S. patent application Ser. No. 08/970,558, filed on Nov. 14, 1997, and incorporated by reference herein. One way of overcoming these drawbacks to manufacturing carbon-carbon composites is to replace the fiber preform with a graphitizable foam that would result in significantly lowered manufacturing and processing costs. The foam preform should have predominantly and uniformly open cells to allow for the subsequent densification of the foam preform. The foam preform should have the necessary mechanical integrity and be physically robust for ease of handling during subsequent processing. The thermo mechanical properties (e.g. thermal conductivity/diffusivity) of a carbon-fiber preform can be attained by a foam preform only if it is graphitized. Therefore, the foam preform must be graphitizable. If these requisite properties are accomplished, a further benefit can be attained with the use of a graphitizable foam preform as opposed to a carbon-fiber preform. A graphitizable foam preform will have the requisite physical and thermo-mechanical isotropy. Carbon-fiber preforms (made from both random fiber and non-woven methods) are profoundly anisotropic in that properties such as thermal conductivity vary according to the orientation of the fibers. The graphitizable foam preform, which does not include fibers, reduces the degree of bulk anisotropy and would be very useful in the development of carbon-carbon composite discs for applications such as friction discs in aircraft brakes.